The present invention relates generally to the field of signal amplification circuitry, such as circuitry used in medical diagnostic systems, and stability techniques used to enhance performance of such amplification circuitry. More particularly, the invention relates to the decoupling of receive coils to reduce crosstalk between signals originating in phased array and other coils.
Magnetic resonance imaging systems have found increasing applicability for a variety of imaging tasks, particularly in the medical field. Such systems typically include coil assemblies for generating rf magnetic fields used to control and excite spin systems in a subject of interest, such as in soft tissues of a patient. A body coil is typically employed for generating a highly uniform rf magnetic field transverse to the direction of the main, DC, magnetic field. A series of gradient coils generate spatially varying magnetic fields to select a portion of the subject to be imaged, and to spatially encode sensed signals emitted by unitary volumes within the selected slice. The gradient fields may be manipulated to orient the selected image slice, and to perform other useful imaging functions.
Sensing coils are employed in conventional MRI systems and are adapted to the particular type of image to be acquired. Such sensing coils are highly sensitive to emissions from the subject positioned within the primary and gradient fields. Such emissions, collected during data acquisition phases of imaging, serve to generate raw data signals which may be processed to extract information relating to the nature and location of different tissue types in the subject. Where the region to be imaged is relatively small, a single channel surface coil may be employed. For example, a linearly polarized shoulder coil is typically employed for producing images of a human shoulder. For larger images, large single coils may be employed, or multiple coils may be used, such as in xe2x80x9cphased arrayxe2x80x9d arrangements. However, the use of large surface coils tends to result in lower signal-to-noise ratios in the acquired image data. Phased array coil assemblies are, therefore, commonly employed to produce images of larger areas, while providing an acceptable signal-to-noise ratio.
In a typical phased array arrangement, several adjacent coils are provided for receiving the signals emitted by the spin systems of interest during the signal acquisition phase of imaging. In phased array coil systems, output signals from each of several adjacent coils are independently amplified by preamplifiers prior to processing of the signals for generation of the image data.
A problem arises in such systems from noise coupling, both between adjacent and non-adjacent receive coils in the array, that lowers the signal-to-noise ratio attained. Additionally, receive coil coupling can lead to artifact propagation. In particular, non-linearities between the homogeneous and gradient magnetic fields may produce an intense localized signal. If this intense signal is close to an active array element, the associated energy may be transferred to other elements of the array, thereby causing the artifact to propagate into the imaging volume. Both the general problem of noise coupling and the specific problem of artifact propagation can be eliminated by proper decoupling of the phased array receive coils during the receive period.
Adjacent coils may be geometrically decoupled by overlapping the coils. Due to the current-carrying paths established by each coil, such overlapping reduces or cancels mutual inductive coupling between the coils, thereby reducing noise coupling. For non-adjacent coils, preamplifier decoupling may be employed. Preamplifier decoupling turns one of the series capacitances of the coil into a parallel resonator network by applying a parallel inductor in series with a low input impedance preamplifier. The parallel resonator thus formed has high impedance, preventing current flow in the main body of the coil, and thereby providing zero mutual inductance with neighbors.
Decoupling is further complicated in the case of a quadrature phased array in which each element of the array is comprised a quadrature coil pair with orthogonally oriented rf magnetic fields. One common quadrature phased array element consists of a loop/butterfly pair in which the loop element is typically sensitive to fields perpendicular to the coil plane and the butterfly element is typically sensitive to fields parallel to the coil plane. Other quadrature coil pairs exist however. The quadrature phased arrays are sensitive to circularly polarized fields since one coil of the element is typically sensitive to fields perpendicular to the coil plane while the second coil is typically sensitive to fields parallel to the coil plane and the two coils are driven 90 degrees out of phase.
In the case of a loop/butterfly pair, adjacent quadrature coils are decoupled by geometric means, i.e. overlap. Non-adjacent butterflies do not need to be decoupled because the butterflies have a RF magnetic field that drops off quickly outside the coil region in the direction parallel to the main DC magnetic field, i.e. the Z-axis. The distance between non-adjacent butterfly loops therefore typically provides good signal isolation. The distance between non-adjacent loops, however, is typically insufficient to provide good signal isolation and, therefore, preamplifier decoupling is used to achieve signal isolation in conventional systems.
If the low input impedance preamplifier is not integrated into the coil, the readout signals from the loop/butterfly pair are sometimes combined by a 90 degree combiner which shifts one signal 90 degrees before adding it to the other signal before amplification. If a low input impedance preamplifier is applied in series with the output from the combiner, the preamplifier will provide decoupling for only one coil of the quadrature pair since the signal from the other coil is phase shifted by 90 degrees and therefore the low input impedance of the terminating preamplifier will look like an open circuit on the coil side of the combiner. The present invention addresses one or more of the problems discussed above.
The invention provides a novel technique for decoupling quadrature phased array elements for use in a magnetic resonance imaging system using low input impedance preamplifiers and combiners. The technique permits the loop and butterfly signals to be combined such that the loop signal is phase shifted 180 degrees, resulting in no impedance transformation. The combiner typically consists of four sections connected in a bridge configuration. Sufficient low impedance is maintained looking from the loop element through the combiner at the low input impedance preamplifier such that the decoupling functionality of the preamplifier is maintained for that loop element. Looking from the preamplifier through the combiner at the coil elements, 50xcexa9 is maintained if both coil elements are matched to 50xcexa9 such that noise figure of the preamplifier is minimized.
In accordance with one aspect of the present invention, a system is provided for decoupling receive coil signals. The system possesses a MR scanner including a receive coil assembly comprising quadrature coil pairs. The system also possesses a control and acquisition circuit connected to the scanner which comprises a coil control circuit and a data acquisition circuit. The system also possesses one or more system control circuits and an operator interface station.
In accordance with another aspect of the present invention, a system is provided for decoupling receive coil signals. The system possesses a phased array coil assembly comprising elements of butterfly and loop coils. The system also possesses a combiner connected to each element which receives a signal from the butterfly coil and the loop coil and which generates an output signal comprising a butterfly coil component shifted 90 degrees and a loop coil component shifted 180 degrees. The system also possesses a preamplifier connected to the combiner which receives the output signal.
In accordance with another aspect of the present invention, a combiner is provided for combining receive coil signals. The combiner comprises four sections which are the equivalent of one-quarter wavelength long transmission lines with different characteristic impedances which are connected in a bridge configuration in such a way that the combiner shifts one input signal by 90 degrees and a second input signal by 180 degrees.
In accordance with another aspect of the present invention, a combiner is provided for combining receive coil signals. The combiner comprises two equivalent inductances and two equivalent bridge capacitances connected as a loop with the two inductances disposed such that each inductance is adjacent to each bridge capacitance. Due to this configuration, a signal which enters the combiner at one corner and exits through the diametrically opposed corner traverses an inductance and a capacitance.
In accordance with another aspect of the present invention, a method is provided for combining receive coil signals. The method comprises transmitting a butterfly coil signal and a loop coil signal from the phased array coil assembly to a combiner which combines the two signals to form an output signal. The combiner combines the signals in such a way that the butterfly coil signal is phase shifted 90 degrees and the loop coil signal is phase shifted 180 degrees. The output signal is then processed in a low input impedance preamplifier to decouple loop coil signals.
In accordance with another aspect of the present invention, a system is provided for decoupling receive coil signals. The system includes a means for transmitting a butterfly coil signal and a loop coil signal from a phased array coil assembly to a combiner. The system also includes a means for combining the butterfly coil signal and the loop coil signal to produce an output signal in which the butterfly coil signal is phase shifted 90 degrees and the loop coil signal is phase shifted 180 degrees. The system also includes a means for processing the output signal such that the loop coil component is decoupled from other loop coil signals.